Combined discriminator and carrier filter circuits



May 1, 1945- M. G. CROSBY 2,374,735

COMBINED ADISCRII\III\IA'IOR AND CARRIER FILTR CIRCUIT Filed March 5l, 1943 4MM/raaf 4,1 T1012 xm' Patented May 1, 19.45

UNITED STATES PAT sur orifice I 2,374,735

COMBINED' DIS CRIMINATOR AND` CARRIER FILTER CIRCUITSEv Murray G. Crosby, Riverhead, `N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application March 31, 1943, Nov. 481,244

6 Claims. C1. Z50-20) My present invention` rela-tes to combined frequency discrimina-tor and carrier energyfilter'v networks, andv more especially toan improvement ina piezo-electric crystall discriminator and car-` rier filter. l

One of the main objects of my present invenl Another important object of my invention is to provide a crystal detection circuit for phase inodeulated- (PM hereinafter for ,lzirevit-y)A carrier waves, or amplitude modulated (AM hereinafter for brevity) carrier waves, the detection circuit having a relatively sharp frequency deviation-rectied output characteristic; the crystal prior tothe demodulatona certain-,portion ofthe.y

dscriminator section of the detection circuit bel ing constructed to have a hat-toppedA responsey characteristic at its output terminals whereby filtered carrier energy may be supplied' with uniform response over a range of frequency shift.

Another object of my invention jis to improve the action of carrier exaltation receivers by constructing the carrier supply source as a ilter network having a pair of piezo-electric .crystalsv equally mistuned to opposite sidesof a predetermined reference frequency whereby the response characteristic at the filter output terminals is band pass with a substantially fiat top'.

Still other objects of my invention are to im.-`

prove the reception of carrier-exalted receivers, and to. provide improved circuits capable of economical and ready manufacture and' assembly.

The. novel features whichI lbelieve to be characteristic of my invention are set forth with particularity in the. appended claims; the invention itself, however, as to both its organizationy and method of .operation will best be understood by reference to the following description, taken in.

connection with the drawing, in which I have indicated diagrammatically several circuitorganizations whereby my invention may be carried into effect. I

In the drawing:

Fig. lshows an embodiment of theinventiom Figurev 4 illustrates:theAlil-C` characteristic provided .by either of thecilzcuits` offFigs.. 1' and..

Referring now to accompanying. drawing, wherein like reference. characters inthe vdifferent figures designate similar circuit elements, there is shown-inlEig. 1 that portion ofA a-combined carrier lter network and: AFC .circuit which issucient for an understanding ofjmy inventionf. It is first pointed out `that in. the. reception; of either carrier wave energy or AM carrier-wave energy, it is often` desirable to provide carrier exalta.;-A tion. As. those skilled in the art fully' lmow,`car rier. exaltation: isl employed` to.A overcome` the, ef,- fects=of selective fading. Inza carrier-exalted IC- ceiver, and; at. some point lll the; receiving'.

modulated carrier.l wave energy issulojected to@ a;

lteraction so.y that. the modulation Sido bands canape removed, andthe resulting filtered carrier energy is employed.

The resulting. relatively pure carrier 'coerce io usually injected into` the demodulator circuit. such a manner thatY any relative .change in omputude whiohmay exist between tho' oorrio and,

modulation sido bonds isf efeotivoly overcome. by the.` mieoted pure carrier enorey- This process has` been generically indicated osl Carrier @rollio-` tion. It is importantthat the selective char# act-ensues of the carrier flltor Ibo such. as to odo-` ouately reieot. tho side, bands.. The System ofv the present invention makes; userof a pair ofl piezo?- electriczcrysta-ls to produce alband pass filter hav.- ing a. greater degree `of side band reiectionl than prior' singler crystal circuits.

g with Il-'ly invention. a piezo-,electric crystallter Fig; 2 illustrates the response characteristics of.

the carrierr filter network,

Fig. 3 shows a modified form of circuit,

network which. is. capableof substantially retrievine all modulation sido bonds from the carrier,

but which lter has a substantially flat-topped;

band pass characteristic at the output terinfirialsy thereof. Referring` to Fig... .1, the piezo-electric crystal elements of the oorrior ltor network are indicated :by numerals. l and l.` Each pf these crystals is mounted between the usual pair of electrodes. A comi-hon resonant input circuit 3- feeols the modulated carrier wave energy tofthese crystals.. Hence. the. input electrode of Crystal l vis. connected to'ooe'side of. the input circuit 3; while. theinput electrode of the crystaly .21 .is .oor-14 nectedto the opposite. n circuit. Y

The inout circuito employs the .Secondary winding `@of input transformer 5: The tun-.ins

condenser Gils oonnootedi shunt With the Windsa ing. 4 .so astoftuno the inputforoult to .a desired Accordingly, there isy provided', in`yaccordance.`

siffle of theV resonant inout predetermined operating frequency. The aforesaid predetermined operating frequency, in the case of a superheterodyne receiver, will be the intermediate frequency (I. FJ. The primary winding 'I is shunted by its own tuning condenser 8, and the circuit l-S is also tuned to the I. F. value. It is not necessary to disclose the circuits of a superheterodyne receiver between the usual signal collector device and the primary circuit of transformer 5. Those skilled in the art are fully acquainted with superheterodyne receiver construction. Where the received signal energy is AM carrier wave energy, there will be provided a branch connection from the circuit 'If-8 to a' demodulator network of any well known type, such as that shown in my U. S. Patent No. 2,063,-

588, issued December 8, 1936. In such case the filtered carrier output of crystals I and 2 will be i fed to such amplitude modulation detector.`

Where the received signal energy is PM carrier wave energy, then the circuit S--l may be preceded by an'amplitude modulation limiter so as to suppress any amplitude modulation effects. Here, again, a branch circuit will lead from the circuit 'I-8 to a demodulator which may take the form of any well known PM detector circuit. In this case, also, the filtered output energy of the crystals I and 2 will be injected into the PM detector circuit to secure the advantages of carrier-exalted reception.

In the case of AM wave reception, or PM wave reception, the circuits 'I--B and lI-I are each tuned to the mean, or carrienfrequency Fc of the applied signal energy. Since it is desirable to provide AFC voltage as well from the network the crystal filterI is additionally constructed in the` manner of a frequency discriminator. Hence, it will be responsive to frequency shifts of the sig-` nal energy with respect to the predetermined fre-` frequency-amplitude characteristics of crystals I and 2. It will be seen that they have sharp peaks at their resonant frequencies F1 and F2. The cross-over frequency of the two characteristics is Fc. The dottedline curve B in Fig. 2 illustrates the over-all response characteristic at the output terminals of the crystals I and 2. The output electrodes of the crystals I and 2 are connected together by resistors 9 and III. The

carriery energy is derived from the crystal filter network. It is this filtered carrier energy which is supplied to the receiver modulator (not shown) for carrier exaltation. At the output terminals of the crystal filter network the 'frequency-amplitude characteristic is the dotted line characteristic B of Fig. 2. It will be seen that the amplitude of the pure carrier energy will be substantially uniform between the limiting frequencies F1 and F2. By locating these limiting frequency values at a reasonable frequency spacing from Fc, the

filtered carrier energy may shift in frequency without any substantial change in amplitude thereof. Furthermore, the band pass action produced effects a high degree of Aside band rejection as is evident from the flanks of curve B.

Since the crystals land 2 are equally and oppositely mistuned with respect to the mean frequency Fc of the applied signal energy, AFC voltage is readily supplied by connecting a pair of opposed rectiers to the output terminals of the crystals. These opposed rectiiiers are diodes, and

are designated by numerals I6 and I'I respectively. The anode I6 of rectifier I6 is connected to the output electrode of crystal l, while the anode Il ofrectifier I'I is connected to the output electrode of crystal 2. The cathodes of both rectiers are connected together by the series-arranged load resistors I8 and I9. The junction of resistors I8 and I9 is connected to the junction of the return resistors Sand I0. Condensers I8 and I9 respectively shunt the load resistors I8 and I9 to bypass intermediate frequency currents. The cathode of rectifier I1 is grounded, andthe AFC bias is taken off from the cathode end of resistor I8.

The functioning 4of the crystal network as a discriminator will be readily understood by those skilled in the art. Sin'ce the two crystals are respectively tuned to frequencies equally and oppositely displaced from the center frequency of the applied signal waves, it follows that a shift of the mean frequency ofthe applied signal waves from the predetermined center frequency Fc will result in the development of relatively increased rectified voltage' across the load resistor of that rectifier which is connected to the crystal whose resonant frequency is located in the direction of the aforesaid frequency shi-ft. In other words,if the mean frequency of the applied signal waves shifts in the direction of F1, then the rectified junction of these two resistors is connected by lead II to the midpoint of winding Il.

|Ihe capacity existing between the electrodes of each crystal is neutralized by voltage fed through f condenserA I5 is connected tothe output electrode I4 andi of crystal 2. The -junction of Condensers I5 is the point from which the substantially pure voltage developed across load resistor I8 will be greater than that developed across load resistor I9. Hence, the'AFC bias will be the differential resultant of the two voltages, and will be essentially positive with respect to ground.

Furthermore, the magnitude of the positive po= tential will be a function of the extent of frequency shift. yOri the other hand, if the frequency shift is in the direction F2 then the rectied voltage across resistor I9 will be greater, and the AFC bias will be essentially negative relative to ground. In Fig- 4 I have shown a calculated discriminator characteristic. AIt will be seen that the peak rectified voltage is developed- 50 cycles apart. 'Ihere would be developed linear AFC bias over the range of 50 cycles. Beyond these peak frequencies the AFC bias decreases very sharply.' It will be clear that the type of characteristicshown in Fig. 4 is sharp, and will tend to control the local oscillator of the receiver quite rigorously.

`my frequency shift fro-rn Fc will result in AFC l bias which will be. applied to the local oscillator in the. usual manner to overcome thefrequency shift. I t will, also, 'be appreciated that if the frequency spacing between F1 and F2 is 50 cycles, then the filtered carrier energy derived from condensers I4 and I5l will be capable of shifting over a range of 50 cycles without;` change in amplitude. Of course, my invention contemplates any frequency spacing between the peak frequencies of crystals I and 2. Furthermore, the'condensers I4 and I5 are adjusted to give equal outputl from each crystal. This would make them approximately equal in magnitude, but if there were amplitude differences in they `crystal filters compensation would be possible with `these condensers. The two crystal filters areseparately neutralized, as previously explained.v kIn this respect the circuit has an advantage over the usual band pass circuit in that line-up is simplified; that is, there is a separate detector connected to each filter section to indicate whenproper neutralization is achieved.

In Fig. 3 I haveshown a modification wherein the neutralizing condensers I2 and I3.may be dispensed with. In this case, the crystals are positioned between crystal holders of thethreeelectrode type. Thus,y the crystal I has'on one face thereof a single electrode which is connected to the low potential side of input circuit 3. The opposite face of the crystal is provided with a pair of spaced electrodes 2| and `22. Electrode 2I is connected tothe high potential side of input circuit 3. The second electrode 22 is connected to the anode'l of rectifier I8. An Y electrostatic shield is provided between the electrodes 2l and 22, as indicated by the vertical dotted line .extending from the grounded electrode 20 of crystal I. Electrode 22 is the output electrode. The crystal 2 has its single electrode on one face thereof designated by numeral 23, and the latter is connected to the high potential side of input circuit 3. The opposite face of crystal 2 is provided with the spaced electrodes 24 and 25.

The electrode24 is connected to the low potential side of input circuit 3. the output electrode of the crystal, and is connected to the anode I'I of diode II. Crystal 2 is, also, provided with an electrostatic shield which is indicated by thev dotted line. The output condensers I4 and I5 are connected to the crystal output electrodes 25 and 22 respectively. The essential diierence between the circuit of Fig. 3 and that of Fig. 1 resides in the fact that neutralization of the crystal inter-electrode capacities is provided by means of the specific threeelectrode construction. Otherwise, the functioning of this circuit is precisely the same as has been explained in connection with Fig. 1. In other words, in the system of Fig. 3 the crystal I feeds its filtered output to Arectifier I6, While the crystal 2 feeds its filtered output to rectifier I'I. Any frequency shift of the carrier relative to Fc will be quickly compensated for by the AFC bias.

While I have indicated and described'several 4 .systems for carryingl my invention into effect, it

will be apparent to one skilled in the art that my invention is by no means limited to' the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

The electrode 25 isI What I claim is:

1. In combination with a source of vhijgh frequency signal energy, a resonant,Y inputcircuit tuned to a predetermined referencefrequency,I a 1 first piezo-electric crystal element provided with input and output electrodes having'V an input electrode connected to Aone side of the input cir'- cuit, a second piezo-electric crystal element provided with input and output electrodes, s aidsecondv crystal' input electrode being connected to the opposite side of said input circuit, saidjcrystal elements .being respectively tuned to frequency values spaced by relatively small amounts on op posite sides of said reference frequency, means for deriving. from the output electrodes ofv said' crystal elements filtered energy of said `reference frequency, a first rectifier in circuit with said first crystalelement to rectify solely the 'ltered output energy thereof, a secondv rectifier in circuit with the second crystal element to rectify solely the filtered output energy thereof, and a common output circuit connecting said rectiers i in polarity opposition thereby to provide' a differential rectified output voltage whose polarity and magnitude depend respectively upon thedirection and extent of the frequency shift of said applied signal energy from the said reference frequency.

2. In combinationk with a source of phase modlated carrier wave energy, a crystal filtered network for providing substantially pure carrier energy, said network including a pair of piezo-electric crystal elements provided with respective input and output electrodes, said crystal elements having peak frequencies respectivelyequally mistuned with respect to a predetermined reference frequency equal to the center frequency of applied phase modulated 'carrier Waves, means coupled to the output electrodes of said crystal elements for deriving therefrom said substantially pure lcarrierwenergy, and said peak frequencies being sufciently close together to provide a response characteristic at the output electrodes of said crystal elements which is a substantially flattopped, band pass characteristic whereby the substantially pure carrier energy may reasonably shift in frequency ,without change in amplitude I and whereby, in addition, there may be effected a high degree of side band rejection.

3,l In combination with a source of p hase modulated carrier wave energy, a crystal lterunetwork for providing filtered carrier energy, said network including a pair of piezo-electric 'crystal elements whose peak frequencies are respectively equally mistuned With respect to the center frequency of applied phase modulated carrier Waves, means coupled to the output electrodes of said crystal elements for deriving therefrom said ltered carrier energy, said peak frequencies being spaced by a. slight amount thereby toprovide a response characteristic at the output electrodes of said crystal elements which is a substantially flat-topped, band pass characteristic whereby the filtered carrier energy may reasonably shift in frequency without change in` amplitude, arst rectier connected in circuit. with one of said crystal elements to rectify solely the filtered carrier output energy thereof, a` second rectier connected in circuit with thesecond crystalelement to rectify solely the ltered carrier output I energy thereof, said rectifiers being connected in opposition, anda commonrectied voltage out put circuit connected to said opposed rectiflers to provide a differential rectied voltage whose magnitude and polarity are dependent respectively i upon the extent and direction of frequency shift of the center frequency of the applied modulated wave energy from the mean frequency of said crystal peak frequencies. f

4. In combination with a source of amplitude modulated carrier energy, a resonant input circuit tuned to the carrier frequency of applied energy, a first piezo-electric crystal provided with an input electrode connected to the input circuit, a second piezo-electric crystal provided with an input electrode connected to said input circuit, said crystals being respectively tuned to p frequency values equally spaced by a relatively small amount on opposite sides of said carrier` frequency, a first rectifier in circuit with said first crystal to rectify solely the filtered output energy thereof, a second rectifier in Ycircuit with the second crystal to rectify solely the filtered output energy thereof, and a common output circuit connecting said rectiers in polarity opposition.

5. In combination with a source of modulated carrier wave energy, a network including a pair of piezo-electric crystal elements whose peak freacteristic.a separate rectifier coupled to a respective crystal element to rectify solely the filtered carrier energy thereof, and a common rectiiied voltage output circuit connecting the rectiilers in opposition. v v

6. vIn combinationl with a source of amplitude modulatedA carrier Wave energy, a crystal filter network for providing substantially pure carrier f energy, said network including a pair of piezoelectric crystal elements Whose peak frequencies v are respectively relatively slightly mistuned with respect to the mean frequency of applied modulated carrier Waves, means coupled to the output electrodes of said crystal elements for yderiving trerefrom said substantially pure carrier energy, the response-characteristic at the output elec` 1 trodes of said crystal elements being a band pass f characteristic whereby the pure carrier energy may reasonably shift in frequency Without substantial change in amplitude, a first rectifier connected in circuit with one of said crystal elements to rectify solely the filtered output energy. thereof, a second rectifier connected in circuit with the second crystal element to rectify solely filtered output` energy thereof, said rectiers being connected in opposition, and a common rectified voltage.output circuit connected to said opposed rectiers to provide a vdifferential rectified voltage Whose magnitude and polarity are respectively dependent upon the .extent and direction of frequency shift of the mean frequency of the applied modulated Wave energy from the mean frequency of said crystal peak frequencies.

' MURRAY G. CROSBY. 

